1. Field of the Invention
This invention relates to refrigeration apparatus and in particular to a defrost control circuit for use therein with redundant switching.
2. Description of the Background Art
In the operation of refrigeration apparatus, such as refrigerator-freezer apparatus, air is cooled by flow thereof in heat transfer association with an evaporator conventionally formed of a plurality of coil turns in closely spaced relationship. As a result of the flow of the air therepast, moisture from the air is deposited on the evaporator in the form of frost. From time to time, it is necessary to remove this frost and maintain efficient operation of the refrigeration system. One conventional method of controlling the defrosting of such an evaporator is to provide timer means effecting a defrosting of the evaporator at preselected timed intervals. In one form, the timer motor is connected in the compressor motor circuit so as to measure time only during the operation of the compressor motor, thereby more accurately correlating the need for defrosting with the actual run time of the apparatus.
Still further, accurate correlation of the need for defrosting is effected by means of a demand control circuit which is responsive to a signal indicating a frosted condition of the evaporator for initiating the defrost operation. Thus, in the demand control system, a sensor is provided for detecting the buildup of frost and initiating the defrosting cycle at the appropriate time.
In one form of defrost system, defrosting heat is provided by a defrost heater which is energized electrically until a defrost bimetallic switch opens to terminate the defrost cycle.
Conventionally, upon completion of the defrost cycle, the circuit is restored to the normal operating condition wherein the compressor is operated in accordance with the cooling demand of the apparatus.
Illustratively, in U.S. Pat. No. 3,013,399 Edward C. Simmons et al disclose a refrigeration apparatus having a demand defrost control means wherein a thermostat switch selectively energizes the compressor or defrost heater. A frost sensor is provided for controlling the thermostat switch which is maintained in the defrost position by means of a bias heater energized concurrently with energization of the defrost heater. A second thermostat switch is connected in the circuit to terminate the defrost cycle and de-energize the bias heater.
A photoelectric ice detecting device is illustrated in U.S. Pat. No. 3,188,828 of Harry A. Wayne. The control utilizes a conventional relay for selectively controlling the operation of the defrost heater and refrigeration apparatus compressor.